It has been found desirable in many instances to provide engines with a leaner than stoichiometric air/fuel mixture. Maintaining a lean air/fuel mixture has the advantage that unburned fuel is not exhausted, reducing the possibility of catalyst fouling and reducing the amount of harmful emissions. While providing an engine with a lean air/fuel mixture is satisfactory in many running conditions, such as when the engine is idling, such does not often provide sufficient fuel to support engine acceleration at other times.
This problem is especially acute in watercraft. As is well known, in order to move a watercraft from a trolling velocity to a planing velocity, a great increase in watercraft velocity must be effectuated. This requires a large increase in power output during a period of engine acceleration. If the engine is supplied with too lean of an air and fuel mixture, the engine will not provide the power output necessary to effectuate planing of the watercraft.
On the other hand, if the amount of the fuel delivered to the engine is increased, if even during only short periods of acceleration, the engine may exhaust unburned fuel components. In modem engines having exhaust systems with exhaust gas catalysts, the exhausting of unburned fuel may result in the fouling or overheating of the catalyst.
A system for providing the required air/fuel mixture to an engine powering a watercraft during all phases of engine operation, and wherein catalyst fouling and overheating is prevented, is desired.